Project Summary The female skeleton undergoes dramatic physiological alterations as a result of reproduction. While weaning induces substantial bone recovery, reproduction-induced bone loss is only partially recovered after weaning. Nevertheless, most epidemiology studies report that history of reproduction and lactation had no negative, or even a protective effect on fracture risk later in life. This represents a paradox that reproduction reduces bone mass without increasing risk of future fractures. Thus, the overall objective of this study is to uncover the mechanisms that explain this paradox. Our preliminary results demonstrated that despite a lower bone mass, reproductive rats have a distinct bone structural phenotype and a much slower rate of bone loss than virgin rats when exposed to estrogen deficiency by ovariectomy (OVX). 3 months after OVX, the load bearing sites, such as vertebral trabecular bone and femoral midshaft, showed no bone loss in reproductive rats. Our data also indicated that osteocytes can actively modulate material properties of the peri-lacunar bone matrix during reproduction, which could lead to critical alterations in the micro-mechanical environment of osteocytes, the presumed mechano-sensors in bone. Indeed, our results suggested that post-reproductive rats were more sensitive to in vivo tibial loading than virgin rats. In addition, when subjected to OVX later in life, the size of osteocyte lacunae increased significantly in reproductive rats. Thus, the micro-environmental changes after reproduction may affect the skeleton's sensitivity to mechanical stimuli and impact bone quality later in life. These findings from clinical and animal studies provide a strong scientific premise for our novel, central hypothesis that history of reproduction and lactation causes skeletal adaptation at the structural, material, and cellular levels, which may protect the skeleton from estrogen deficiency-induced bone loss later in life. To test this hypothesis and determine the mechanisms behind it, we propose two aims. In Aim 1, we will determine the influence of reproduction and lactation history on skeletal responses in bone microarchitecture, cellular activities, and mechano-sensitivity to estrogen deficiency later in life. In Aim 2, we will establish the role of reproduction history on modulating lacunar and canalicular structure, peri-lacunar bone tissue modulus, and load-induced fluid flow experienced by osteocytes and their processes in both prior- and post-OVX rats. We will address an unsolved clinical paradox and elucidate the amazing adaptive mechanisms that protect women with a history of pregnancy and lactation from postmenopausal osteoporosis. Identifying the phenotypic bone structure, material properties, and osteocyte microenvironment in virgin and reproductive bone could lead to novel strategies for osteoporosis prevention, management, and treatment for postmenopausal women by considering their reproduction histories. This highly impactful research will also advance our fundamental understanding of osteocyte peri-lacunar remodeling and its regulation of bone's mechano-sensitivity.